1. Field of the Invention
Polypropylene films have found wide acceptance in the packaging industry, especially as a food packaging agent, because of their superior physical properties. Polypropylene film, usually biaxially oriented, is characterized by high tensile modulus and stiffness, excellent optical clarity and good moisture resistance. However, polypropylene film possesses one major disadvantageous property, high inherent coefficient of friction (COF).
High COF complicates the processing of polypropylene film. Polypropylene film processing is impeded by poor transport, caused by its high COF, over rollers, guides and the like. In addition, high COF creates film storage problems. Because of its high COF one layer of polypropylene film sticks to those above and beneath it, creating the problem commonly referred to as blocking.
This serious disadvantage of polypropylene film is well known to those skilled in the art. Thus, many proposed solutions to overcome this major deficiency have been proposed in the art. One such proposal has been to incorporate additives in the polypropylene resin processed to form the film. A favorite additive suggested in the art is fatty acid amides. These amides decrease the film's COF.
Although this method has been used, actual COF is a function of the heat history to which the film has been exposed during shipping, storage and processing. As such, it is subject to wide variation. More significantly, the presence of fatty acid amides on the film surface oftentimes adversely affects the appearance of the film as manifested by decreased gloss and the presence of streaks. Another serious disadvantage of using fatty acid amides is the effect of fatty acid amides on polypropylene film surface wettability and adhesion. This adverse characteristic applies to coatings, inks, adhesives and the like whether organic or water based.
Of even greater concern in the use of fatty acid amides to overcome the problem of high COF resides in the manufacture of the polypropylene film which contains the amide. Fatty acid amides must be applied as an organic solution, utilizing an organic solvent. Health and safety factors thus complicate in-plant utilization of this class of slip additive.
Because of these problems other slip additives have been suggested to overcome the inherent problem of high COF in polypropylene films. One such solution is the use of a latex coating containing stearamidopropyldimethyl-betahydroxyethylammonium nitrate and a crosslinkable acrylic copolymer. This coating has been advanced for use on polyester films.
Although this coating is alleged to improve slip properties without adversely affecting clarity in polyester films it is unnecessary to determine whether this coating would provide the same improved result on polypropylene film. It is known that such a coating is not in conformity with the requirements and regulations of the U.S. Food and Drug Administration for use in films which contact foods. As such a critical market for polypropylene films, as a food packaging agent, could not be exploited if such a material was added to polypropylene-containing film.
Yet another suggestion advanced in the prior art to reduce the blocking characteristic of polypropylene film is to add a finely divided inorganic material as a thin surface layer. Such a coating not only provides a non-blocking surface having improved slip characteristics but is also independent of the heat history of the film. Moreover, the addition of a surface layer of an inorganic material does not create the adverse optical and wettability effects associated with amide-modified films.
However, like the other proposed solutions, this proposed means of alleviating high COF creates new problems. Oftentimes, polypropylene film is laminated to other films. For example, glassine paper is commonly laminated to polypropylene film. Such a laminate, when provided as a thin surface layer containing finely divided inorganic material, exhibits significantly higher COF values than does the unlaminated polypropylene film. Such laminates are known to perform poorly on conventional form, fill and seal machines.
The above discussion establishes the need in the art for a new and improved polypropylene film characterized by improved anti-blocking characteristics and decreased COF. However, it is incumbent that the improvement in the film's slip property not correspond to the decline in other properties, typical of the solutions advanced in the prior art.
2. Background of the Prior Art
There are several teachings in the prior art relating to thermoplastic films in general and polypropylene films in particular which are fabricated, treated or modified to lower film coefficient of friction. In addition, there are disclosures directed to compositions which may be utilized as film coatings to lower film COF.
One of the earliest relevant references is U.S. Pat. No. 3,176,021. This patent describes a process for preparing a polypropylene film having decreased static COF. In this process crystalline polypropylene is admixed with polyethylene and an amide of a water insoluble carboxylic acid. The composition is melt extruded and processed into a polypropylene film having improved slip properties.
U.S. Pat. No. 3,753,769 is directed to an acrylic based polymer composition for use in coating polyolefin films. The coating composition comprises an interpolymer of an alpha,beta-monoethylenically unsaturated carboxylic acid, an alkylacrylic ester and an alkylmethacrylic ester.
A composite film is set forth in U.S. Pat. No. 4,225,644. The film includes a substrate layer of biaxially oriented polypropylene. The laminate composite film further incorporates a middle layer of a uniaxially or biaxially oriented film of an ethylene-butene copolymer. The outer layer of this composite film, which provides decreased COF, is a uniaxially or biaxially oriented thin film of a primary higher fatty acid amide or an N-substituted higher fatty acid amide.
The teaching of U.S. Pat. No. 4,302,506 is concerned with a biaxially oriented polyester film. The film is coated on one or both sides with a stearamidopropyldimethyl-betahydroxyethylammonium nitrate and a crosslinkable methylmethacrylate-ethylacrylate-methacrylamide terpolymer. This coating is recited to produce improved slip properties.
A common teaching in U.S. Pat. No. 4,419,410 and 4,419,411 is to a polypropylene film laminate of a base layer and a skin layer. The base layer of the '410 patent is a comparative high stereoregular polypropylene film containing a surface modifying agent which may be an anti-blocking agent. In the '411 patent the base film is a comparatively high stereoregular polypropylene film which specifically includes an amide or a monocarboxylic acid anti-blocking agent. The skin layer of the '410 patent is again a polypropylene film. The polypropylene of the skin layer is of comparatively low stereoregularity, containing the same surface modifying agents provided in the base film. However, the surface modifying agent of the skin layer is bloomed on its surface in greater concentration than is the surface modifying agent bloomed onto the surface of the base layer. The skin layer of the '411 patent is a polypropylene film also of comparative low stereoregularity which includes finely divided silica and silicone oil. In addition, the skin layer of the '411 patent includes the same amide of a monocarboxylic acid contained in the base film layer.
U.S. Pat. No. 4,486,483 describes an oriented polyester film having a primer coating on at least one side thereof. The coating, said to improve the adhesion of printing inks to polyester film, comprises a water soluble alkali metal salt of an unsaturated fatty acid having from 10 to 18 carbon atoms, an alkyl sulfate having from 8 to 18 carbon atoms or mixtures thereof.
Although a coating of an alkali metal salt of an alkyl sulfate is taught in this disclosure, it is advanced for use only with polyester films to improve ink adhesion. No suggestion is made in this patent that such a coating would improve the slip properties of a polypropylene film which does not include polyester resin.
The disclosure in U.S. Pat. No. 4,533,509 is related to a method of preparing a thermoplastic film free of organic COF reducing agents. In this method a skin layer of an optically clear thermoplastic resin containing inorganic particles is sequentially or simultaneously bonded to a composite thin surface layer of the same thermoplastic resin free of inorganic blocking agents.
U.S. Pat. No. 4,578,316 teaches an oriented film of a blend of polypropylene and polyethylene. The polyethylene constituent of the mixture may be high density polyethylene, medium density polyethylene or a mixture of the two. The film is recited to provide improved COF properties.
A laminate film structure is the subject of U.S. Pat. No. 4,618,527. The laminate film structure of the '527 patent contains a thin film of a thermoplastic resin containing a finely divided inorganic material. The laminate also includes a comparatively thick base layer of a thermoplastic film containing a fatty acid amide.